Fix issue #1616, fileeditor does not close under Mono.

Mono bug https://bugzilla.xamarin.com/show_bug.cgi?id=5040
triggers recursive calls to FormEditor_FormClosing() when setting
DialogResult.  I added a flag to indicate that the form is already
closing which breaks the recursion.
Merge pull request #1617 from chrisbux/fix_1616

Fix issue #1616, fileeditor does not close under Mono.